Conventionally, paper friction materials comprising paper as a base material have been mainly used as wet friction materials. The paper friction materials are produced by mixing various kinds of friction modifiers with pulp, then performing papermaking, and impregnating the resulting material with a bonding resin represented by a phenol resin, followed by curing. The paper friction materials are applied to a wide range of uses because they have an appropriate friction coefficient. However, these days, in order to improve vehicle fuel efficiency and cope with the increased loads put on wet friction materials by increased engine output and increases in vehicle weights, there are demands for further increases in the friction coefficient and further improved durability.
Patent Reference 1 proposes wet friction materials using a phenol resin modified with a drying oil such as tung oil as a bonding resin.    Patent Reference 1: Japanese Patent Laid-Open No. 1-269734